The present invention relates to loudspeaker enclosures, and more specifically, to loudspeaker enclosures which minimize sound output coloration caused by sympathetic cabinet vibration.
Loudspeakers and loudspeaker designs have been well known since the 1940's. Over the past 40 years a great deal of work has been devoted to the design of loudspeakers. They are complex devices. Incorporated in a complete system are a group of dynamic sub-systems integrated into a unified sound producing package. System integration is critical to the absolute performance of the design. One of the most significant yet least controllable sub-systems is the enclosure itself and the potential it has for coloring and distorting the sonic output of the system.
The ideal loudspeaker enclosure would be infinitely stiff and rigid. It would secure the drivers in their fixed positions and contribute nothing to the sonic character of the system. In reality, enclosures are rather elastic structures with dynamic characteristics. Reactions to internal pressures and drive vibration cause the cabinet itself to resonate and produce sonic output. At specific resonant frequencies the output from the cabinet can be as strong as the direct driver output. This sound is chiefly parasitic in nature. It is fundamentally unlike the output of an electro-acoustic transducer, which has a defined and controlled response from an electrical input signal to an acoustic output--or transfer function. Typically, loudspeaker enclosures are formed by the joining of six or more flat panels, each of which exhibit flat plate resonances.
Countermeasures can be taken to try and control the enclosure vibration. These include using select materials, bracing, mass and stiffness enhancement to control panel resonance and vibration, additional damping materials, and driver isolation mounting. Many enclosures are merely soundproof boxes formed of wood composite and filled with a sound absorbent material.
Recently, attempts have been made to reduce enclosure vibration and the corresponding sound output behavior of loudspeaker enclosures. The techniques employed have been attempts to control panel vibrations with extensional damping materials and/or the use of mechanical bracing, tension rods, or trusses.
One approach applies a layer or layers of a high loss bituminous mastic to the inside of the cabinet walls. Vibration energy is dissipated in the damping material through the mechanism of extensional damping. Another approach employs a threaded rod which screws into two opposing internal surfaces of the enclosure. This rod exerts tension on the enclosure and attempts to limit its ability to vibrate. Still another approach using a complex system of braces and trusses attempts to make the mechanical impedance of the enclosure walls sufficiently high at the frequencies of interest so that no motion is imported to the cabinet walls.
These approaches all fail in practice to eliminate significant amounts of enclosure vibration. The addition of damping material attempts to dissipate energy in a moving cabinet wall by changing the kinetic energy into thermal energy. This is impossible to do with any consistency and efficiency. The second approach attempts to stop flexing on six different vibrating panels with a single brace between two of the six. The third approach is inordinately difficult to achieve in practice because the mis-alignment of the brace(s) or a less than adequate mechanical joint(s) greatly reduces the high mechanical impedances required and, in fact, may produce more vibration rather than less.
Additionally, an enclosure formed from flat panels will exhibit a non-uniform radiation loading of the loudspeaker as a function of frequency, resulting in acoustical diffraction. It has been shown in the art that the optimum shape requires compound curves and approaches a sphere.